Thin film titanium nitride (TiN) is widely utilized throughout the integrated circuit industry as a diffusion barrier. A diffusion barrier is an inter-layer between the silicon contacts and the metal inter-connection. Its primary purpose is to prevent junction spiking failures which occur when a significant amount of metal diffuses into the silicon creating a short circuit across the junction. As contact dimensions shrink, this diffusion process is driven by high current density and higher local temperature making an effective diffusion barrier an essential part of the integrated circuit fabrication process.
Titanium nitride is also used as an adhesion layer or blanket tungsten films. In this application titanium nitride is deposited after contacts or vias are cut in the dielectric. Blanket tungsten is then deposited and etched back to form plugs which are coplanar with the top of the dielectric. Then aluminum is deposited and patterned to form the metal interconnection for the integrated circuit. This series of processes is usually repeated to form three or four levels of metalization.
There are three processes for depositing titanium nitride films. These are sputtering titanium onto a substrate and then reacting in nitrogen or ammonia, reactively sputtering titanium in a nitrogen ambient and chemical vapor deposition (CVD). The first two processes are physical and result in line of sight trajectories for the deposited material. As a result, coverage of the side walls and bottoms of high aspect ratio contacts is poor with respect to the top surface of the substrate. The third process, CVD, allows surface diffusion of the depositing species and so the coverage on the side walls and bottoms of the high aspect ratio contacts can be equivalent to that on the top surface of the substrate. An apparatus and method useful for such chemical vapor deposition of titanium nitride films is disclosed in pending applications Methods of Chemical Vapor Deposition (CVD) of Films on Patterned Wafer Substrates, Ser. No. 07/898,492 filed Jun. 15, 1992 and Semiconductor Wafer Processing Method and Apparatus With Heat and Gas Glow Control, Ser. No. 07/898,800 filed Jun. 15, 1992, the disclosures of which are incorporated herein by reference.
The excellent conformality which has been demonstrated by chemical vapor deposition from titanium tetrachloride and ammonia is usually accomplished at a temperature of 650.degree. C. However, substantial benefits could be realized from this process if the deposition temperature could be reduced to less than 550.degree. C. Reducing the deposition temperature to less than 550.degree. C., and preferably 450.degree. C., would make the deposition process compatible with aluminum metalization. A low temperature process such as this could be utilized not only to deposit diffusion barriers at the contact level, but also to deposit adhesion layers for blanket tungsten deposition at subsequent metalization levels without disturbing the underlying aluminum layers. There are also other metalization schemes which require a low temperature titanium nitride process.
In CVD deposition of titanium nitride, the reaction rate versus the reciprocal of temperature appears as a graph as shown in FIG. 1. This is also referred to as the Arrhenius plot. This graph shows two basically linear lines, a horizontal line which represents higher temperatures from about 600.degree. C. and higher, and a sloped portion from 600.degree. C. and lower. This horizontal portion is called the mass transfer portion where the deposition rate is limited by the mass transfer. The sloped portion is limited by the reaction rate. In this region, the reaction rate for titanium nitride deposition can be expressed by the following equation: EQU R=3.48.times.10.sup.-7 exp(-4800/T)P.sup.0 TiCl.sub.4 P.sup.2 NH.sub.3
There are two problems which occur with chemical vapor deposition of titanium nitride at the reaction rate limited temperatures. The first of course, is the reaction rate itself. This can be slow, increasing deposition time. Also, and more importantly, at these lower reaction temperatures chlorine impurities remain in the deposited film. The chlorine impurities increase the resistance of the titanium nitride film. Also, the chlorine present in the deposited film corrodes metal, in particular aluminum, damaging the surface.
In a rotating reactor such as disclosed in co-pending application Ser. No. 07898,492 entitled Method of Chemical Vapor Deposition (CVD) of Films on Patterned Wafer Substrates filed Jun. 15, 1992, it is known that the reaction rate in the mass transfer region of the Arrhenius plot can be increased by increasing the rotation rate of the disk for certain CVD films. For example, Heterogeneous Kinetics and Mass Transfer and Chemical Vapor Deposition Crystal Growth Characterization, 1981 Vol. 4, pp. 283-296, discloses this phenomenon with respect to CVD deposition of tungsten silicon chloride. However, their findings showed that there was no increase in reaction rate in the reaction controlled portion of the Arrhenius plot for the deposition of Tungsten Silicon Chloride. Thus the rotation rate had no effect on the reaction rate at lower temperatures.